Electrical protective device



' w. R. BAKER ELECTRICAL PROTECTIVE DEVICE May 6, 1958 Filed May 17,1956 Y L P P U s INVENTOR.

WILLIAM R. BA KER 0 o BIAS POWER SUPPLY 45 ATTORNEY.

United States Patent ELECTRICAL PROTECTIVE DEVICE William R. Baker,Orinda, Califi, assignor to the United States of America as representedby the United States Atomic Energy Commission Application May 17, 1956,Serial No. 585,588

Claims. (11. 317-51 The present invention relates to an improvement inthe field of protective devices for electrical apparatus and isparticularly directed to the discharging of resonant cavities forpreventing energy back flow therefrom through associated equipment as aresult of faults in the latter.

Although various applications of the invention are possible, particularadvantage has been achieved in employing the invention in connectionwith particle accelerators wherein resonant cavities or waveguides areenergized to high levels for establishing particle accelerating fields.In such devices wherein large current beams of charged particles areoperated upon, it is necessary to supply very large quantities of highfrequency power to the cavities, or waveguides as the case may be, andin this operation high power discharge tubes are employed. Operatingfaults in the power supply to the cavities, such as sparking in thedischarge tubes, provide a discharge path into which the cavity wouldunload, thereby causing very material damage, such as destruction of asparking tube.

Although innumerable protective devices have been developed andmarketed, it has been found that none capable of handling loads of themagnitude herein encountered operate with sufiicient rapidity to aflordrequisite protection. The present invention operates in the hereinpreferred embodiment to discharge a resonant cavity or waveguide in anextremely short time following a fault in the means supplying energythereto and thus protects the latter from cavity discharge therethrough.

It is an object of the present invention to provide an improvedprotective device for discharging very high power upon the occurrence offaults in equipment being protected.

It is another object of the present invention to provide a dischargedevice for producing a protective controlled vacuum discharge.

It is a further object of the present invention to provide an improveddischarge device operating wholly electrically to substantiallyinstantaneously discharge high voltage electrodes in vacuum.

It is yet another object of the present invention to pro vide protectivearc discharge means rapidly unloading cavities.

It is a still further object of the present invention to provide meansfor discharging the energy in a resonant cavity substantiallyinstantaneously upon reverse conduction in power supply means energizingthe cavity for protecting such power supply means.

Numerous other advantages and possible objects of the invention willbecome apparent from the following description of a single preferredembodiment of the invention taken together with the attached drawingillustrating this embodiment. The sole figure of the drawing illustratesone embodiment of the invention including mechanical structure andassociated electrical cincuitry.

The invention in general provides means defining a spark gapcommunicating with the interior of a cavity or waveguide adapted forhigh power energization or an a high vapor pressure, such as copper.

2,833,965 Patented May 6, 1958 ice evacuated chamber containing anelectrode having a large power difierential from the wall or otherelectrode therein. A control or trigger circuit is connected between apower supply energizing the cavity and the spark gap whereby reversecurrent flow in the power supply circuit instantaneously triggers thespark gap to initiate discharge within the cavity, whereupon cavityenergy discharges across the gap, or with an electrode present, theelectrode discharges to one of the spark gap elements.

Considering now the illustrated embodiment of the invention in somedetail and referring to the drawing, there is shown in part a resonantcavity or waveguide 11 defined by one or more walls 12 having an innerconducting liner 13. This cavity 11, which may also be termed a tank,may be quite large, as of the order of many feet in diameter, used forexample in charged particle accelerators, and may have any desiredconfiguration, such as cylindrical. In the particular cavityillustrated, the liner 13 is electrically grounded and the cavityenergized from a coupling loop 14 mounted in the tank 11. The loop 14 isconnected outside the tank 11 to a cavity power supply 16 via alead-through insulator 17 in the tank wall 12 and liner 13. It iscontemplated that upon operation of the power supply 16 there will besupplied high frequency electrical power to the coupling loop 14 withinthe tank whereupon such energy shall be coupled into the tank in aconventional manner to excite the cavity 11. The cavity excitation maybe employed as desired, as by utilizing the electric fields establishedto accelerate charged particles.

In particular cavity applications, such as the one suggested above, verylarge amounts of power are fed into the cavity via the coupling loop. Itwill be appreciated that the coupling loop 14 will couple energy intothe cavity and also will couple energy out of the cavity, although notnecessarily with the same efficiency, so that a large amount of powerwould be fed back from the cavity through the loop to the power supplyunder certain conditions if a complete circuit was provided therefor.This would, of course, be an inefficient procedure and conventionalpower supplies as herein contemplated include a unidirectionalconducting device in the output, i. e., a high power vacuum tube. Undervery high power operation vacuum or gas tubes subjected to large reversevoltages may fail to hold these voltages and spark or fire in a reversedirection. In the present instance such high voltages are present and doappear across the power supply output. In this case with very largeamounts of power stored in the cavity, the provision of a reversecurrent circuit through the power supply would result in an extremelylarge reverse current flowing from the cavity through the power supplyto ground. Such a discharge would not only be wasteful of power butbecause of its magnitude would be very dangerous in that the reversefiring tube in the power supply would carry such high currents as to bematerially damaged and probably destroyed, quite possibly with anexplosion of the tube affected.

The present invention operates to discharge or unload the cavity 11under conditions described above and to this end there is provideddischarge means 21 including as a part of the structure thereof an outercylindrical electrode 22 mounted in the tank wall 12 and extendingthrough the liner 13. The electrode 22 is hollow and the inner endthereof is flared at least inwardly to form a small circular opening.Concentrically within the cylindrical electrode 22 is disposed a centerelectrode 23 having a flared end aligned in a plane with the flared endof the outer electrode 22 to form a small annular space or gap 24thereoetween. The electrodes 22 and 23 are metallic with one of theelectrodes formed of tungsten, for example, and the other formed of amaterial having In the instances where the electrodes are associatedwith high power cavities they are formed with substantial dimensions tohave sufficient structural strength to accommodate high power arcingtherebetween without material damage.

The outer electrode 22 may be mounted in any suitable manner in the tankwall 12 with provisions for removal if expected usage is such as toindicate wear requiring parts replacement. The inner electrode 23 isadjustably mounted in the end of a rod 26 threaded into a large nut 27secured in a plate 23. The plate 23 is mounted at a distance from thetank wall 12 by a number of stand-off insulators 29 which may serve notonly to insulate the plate but also to support same and to fix theposition thereof. A second rod 31 also threaded into the nut 27 mayserve as a terminal for electrical connection of the center electrode23, the outer electrode 22 being electrically grounded by connection tothe tank wall The cavity 11 is evacuated and protection of the vacuumthereof at the discharge means 21 may be accomplished by provision of abell housing 32 over same and sealingly connected to the tank wall 12and to the terminal rod 32.

Operation of the above-described discharge means 21 is controlled by acontrol circuit as which includes a peaking transformer 37 having theprimary winding 38 thereof connected between one terminal of the cavitypower supply 16 and ground, the other power supply terminal beingconnected to the cavity coupling loop 14. The secondary winding 39 ofthe peaking transformer 37 is connected in series with the primarywinding 41 of a pulse transformer 42 across a bias power supply 43. Thepeaking transformer has a saturable core and, in operation, the biassupply 43 passes suflicient current through the peaking transformersecondary 39 to saturate the core whereby nominal current fluctuationsin the primary winding 38 thereof do not produce an appreciable signalin the secondary. A large reverse current in the primary 38 of thepeaking transformer, as results from a reverse short in the powersupply, produces a magnetic flux in the peaking transformer inopposition to the flux resulting from the biasing current and ofsufiicient magnitude to overcome same whereby a large current is inducedin the secondary winding 39. This large current flows through theprimary winding 41 of the transformer 42 thereby inducing a high voltagepulse in the secondary winding 44 thereof. The pulse transformersecondary is connected between ground and the discharge terminal 31which is tied to the inner electrode 23 so that this high voltage pulseappears between electrodes 22 and 23 across the gap 24. As the gap 2 isquite small and as the voltage pulse impressed thereacross is quitehigh, a spark is generated thereacross even through a vacuum. Thisdischarge or spark across the gap 24 immediately is fed by the cavityenergy and the spark expands and draws the energy from the cavity whichunloads therethrough.

It will be seen from the foregoing that any fault or failure whichprovides a short circuit from the coupling loop to ground, or from ahigh voltage electrode storing large amounts of power within the cavity,will cause a reverse current to flow through the peaking transformerreversing the polarity thereof and impressing a large voltage pulse onthe transformer 42 whereby same appears across the gap 24 within thecavity. There is thus produced across gap 24 a spark discharge which isfed by the energy within the cavity to dissipate same before it can dumpthrough the power supply. Although it is quite difficult to initiate aspark discharge in a high vacuum, the small gap distance and highvoltage impressed thereacross, as of the order of 100,000 volts,operates to produce an initial spark that then expands. In the casewhere a central electrode is disposed in the cavity and is to bedischarged the spark actually expands from the original gap over to thecentral electrode. The sparking produces a very rapid energy dischargeof the cavity, requiring only a few microseconds, and thus precludeslarge energy discharge through a faulty power supply.

Although the invention has been disclosed with respect to a singlepreferred embodiment, it will be apparent to those skilled in the artthat numerous modifications and variations are possible within thespirit and scope of the invention and thus it is not intended to limitthe invention except by the terms of the following claims.

What is claimed is:

1. Cavity discharge means comprising means defining a spark gapcommunicating with a cavity and control means connected in circuit withmeans energizing the cavity and producing a high voltage pulse acrosssaid gap under the condition of large reverse current flow from thecavity.

2. A vacuum discharge device comprising an outer cylindrical electrodehaving an inwardly flaring end adapted for connection to an energizedcavity, an inner electrode disposed concentrically within said outerelectrode and having an outwardly flaring end disposed coplanar with theflared end of said outer electrode to define an annular spark gaptherebetween, and a control circuit applying a high voltage between saidelectrodes upon short circuiting of means energizing the cavityassociated with the device whereby a spark discharge is establishedbetween said electrodes.

3. A protective device for cavity energizing means comprising meansdefining a spark gap adapted to communicate with a cavity connected toenergizing means, a biased peaking transformer connected in circuit withsaid cavity energizing means for producing a signal upon reverse currentflow through said cavity energizing means, and a pulse transformerconnecting said peaking transformer and said spark gap for applying alarge voltage across the latter to establish a discharge thereacrosswhereby the cavity discharges upon reverse current flow through saidcavity energization means.

4. A protective device comprising means defining a spark gap andcommunicating with an electrical cavity energized from a power supply tobe protected, a peaking transformer having a primary winding connectedin circuit with said power supply and an element in said cavityinsulated therefrom, means biasing said peaking transformer, and a pulsetransformer having a primary Winding connected to the secondary windingof said peaking transformer and a primary winding connected across saidspark gap for impressing a high voltage across same upon large reversecurrent flow from said cavity element.

5. An electrical protective device for the power supply circuit of acavity resonator including a power supply energizing a cavity resonatorthrough coupling means and comprising means defining a short spark gapadapted to communicate with said cavity, a peaking transformer biased tosaturation connected to said power supply for inducing a voltage thereinupon reverse current flow through the power supply, and a pulsetransformer connected to said peaking transformer and across said sparkgap to apply a voltage pulse thereto for initiating spark dischargewhereby said cavity discharges through the spark.

References Cited in the file of this patent UNITED STATES PATENTS2,763,816 Baker Sept. 18, 1956

